He made the comments in a letter to former chiefs of the U.S. Drug Enforcement Administration. The Associated Press obtained a copy of the letter, dated Wednesday.

“We will vigorously enforce the CSA against those individuals and organizations that possess, manufacture or distribute marijuana for recreational use, even if such activities are permitted under state law,” Holder wrote.

He also said that legalizing recreational marijuana in California would be a “significant impediment” to the government’s joint efforts with state and local law enforcement to target drug traffickers, who often distribute marijuana alongside cocaine and other drugs. Holder said approval of the ballot measure would “significantly undermine” efforts to keep California communities safe.

If Proposition 19 passes in November, California would become the first state to legalize and regulate recreational pot use. Adults could possess up to an ounce of the drug.

California, like 13 other states and the District of Columbia, has legalized medical marijuana. In that realm, federal agents have focused on busting dispensaries they said were using medical marijuana as a front for traditional drug-dealing.

“We will not tolerate drug traffickers who hide behind claims of compliance with state law to mask activities that are clearly illegal,” Holder said in a statement a year ago. A Justice Department memo issued at the time urges federal prosecutors to pursue marijuana cases which involve violence, the illegal use of firearms, selling pot to minors, money laundering or involvement in other crimes.